This invention relates to an expandable work-holding mandrel for use in a variety of manufacturing operations/processes.
Expanding mandrels were initially developed for securely holding and locating a workpiece by its inside diameter in order to perform various types of manufacturing operations on the workpiece. Some of these manufacturing operations include: turning material from the outside diameter of a bushing on a lathe; grinding material from the outside diameter cylindrically, or grinding flats and keyways on a hollow shaft; drilling or boring blind holes, milling flats, keyways, or slots on a workpiece; and performing inspection operations with the mandrel located between work centers.
Traditionally, expanding mandrel designs employ a hardened steel arbor that tapers down gradually along its entire length. A hardened steel sleeve with a matching but slightly smaller inside taper with long opposing slots is pressed over the arbor. As the sleeve is pressed down over the arbor, the sections created by the long slots are forced outward by the interference between the two tapers and away from, but roughly parallel to, the centerline of the tapered arbor. This common type of expanding mandrel, while having been in use for many years, has never been a particularly accurate method for workpiece fixturing and holding. Also, this type of expanding mandrel cannot be easily machined after hardening, and hardening is required for the mandrel to function properly. In addition, this hardening or toughening of the mandrel body makes conventional cross-machining through the workpiece and into the body of the mandrel such as drilling and/or reaming difficult if not impossible.
A number of different approaches for expanding mandrel designs have emerged in recent years. One such type of mandrel uses a series of spacers between which round rubber O-rings are employed. The spacers are fixed at one end of the body of the mandrel, and an actuator (typically a threaded collar or nut) compresses the full assembly toward the fixed end. This linear force causes a compression of the numerous O-rings producing a slight expansion to the O-rings nominal outside diameters. These expanded O-rings in turn capture the inside diameter of a workpiece to both locate and secure it to the body of a mandrel. This type of mandrel requires a very tightly controlled size tolerance for the workpiece inside diameter to function properly. It is also a costly mandrel to manufacture requiring a large number of components, and must also be manufactured from hardened steel to hold up to the high loading forces present during its use.
Accordingly, there is a need for an expanding mandrel that accurately, reliably, and conveniently holds a work piece and is cost-effective to produce.
According to a first aspect of the present invention, it is recognized that expanding mandrels, while varying in design, are conventionally manufactured from hardened steel. This hardening of the mandrel makes conventional machining of the mandrel body a difficult and expensive process using standard tooling. Accordingly, one aspect of the present invention is to provide and expanding mandrel that need not be made from hardened steel, and may be machined by turning the body of the mandrel to a given diameter in a CNC (Computer Numerical Control) or a conventional engine lathe.
According to a second aspect of the present invention, it is recognized that hardened mandrel bodies prevent conventional cross-machining through the workpiece and into the mandrel body such as drilling, reaming, or boring. The present invention provides a mandrel the may be machined using conventional machine tools and operations.
According to yet another aspect of the present invention, a mandrel is provided allowing a very close cylindrical centerline relationship to be maintained between the inside and outside diameters of a workpiece. Consistent with this aspect, an expanding mechanism is provided to hold a true position relative to the centerline of the mandrel body through repeated expansive cycles, reducing typical centerline error by maintaining the functional relationship of the expanding cross sections of the mandrel relative to the centers machined into the end members.
An expandable mandrel is provided including a mandrel body having an interior surface and an exterior surface. The interior surface defines first and second interior regions separated by a transition surface, the first region having a cross-sectional area greater than a cross-sectional area of the second region. A plurality of slots extend between the interior surface and the exterior surface. Each of the slots traverses at least a portion of the first and second regions. A slave actuator having a bearing surface is at least partially disposed in the first region, and a master actuator is provided for urging the bearing surface against the transition surface.
According to a particular embodiment, the mandrel is formed as a tube having a larger inside diameter at each end relative to the inside diameter in a central region of the tube. A plurality of longitudinally oriented slots extend through the wall of the tube, with each end of the tube being circumferentially continuous. A master actuator is disposed in each end of the tube. The master actuator is supported by captive supports in each end of the tube, and drives a slave actuator toward the center of the tube, and into a tapered or rounded transition region between the larger inside diameter at each end of the tube and the smaller inside diameter in the central region of the tube. The linear force of the slave actuator against the transition region produces a radially outward force on the tube, causing the tube wall sections between the longitudinal slots to expand outward, increasing the outside diameter of the mandrel. The captive supports desirably include centering features in outwardly facing surfaces.